Paper adhesive tapes are widely used for masking at the time of every painting on vehicles, buildings, and the like. The paper adhesive tapes are required to have sufficient strength such that they are not easily torn at the time of peeling. As a support of such a paper adhesive tape, there are known ones having a configuration in which a synthetic resin is coated as an undercoat layer of an adhesive layer on one surface of a base paper in which a wood pulp is mixed with synthetic fibers, and a synthetic resin is coated as a sealing layer of a release agent and a paint permeation-preventing layer on the opposite surface (such a support will be hereinafter referred to as “coating type Japanese paper tape base material”). In addition, there are known ones in which a synthetic resin is impregnated in a base paper in which a wood pulp is mixed with synthetic fibers and dried, and a synthetic resin is coated as a sealing layer of a release agent and a paint permeation-preventing layer on one surface of the impregnated paper (such a support will be hereinafter referred to as “impregnation-coating type Japanese paper tape base material”).
The coating type Japanese paper tape base material is restricted on an adhesive in view of a problem of strike through of the liquid in an adhesive coating step because the synthetic resin does not sufficiently exist in the thickness direction of the base paper. In addition, the coating type Japanese paper tape base material is inferior in flexibility as a masking tape to the impregnation-coating type Japanese paper tape base material, and there is involved such a problem that it is hardly stuck onto a curved adherend. The impregnation-coating type Japanese paper tape base material is less restricted on an adhesive, and an aqueous acrylic adhesive excellent in releasability that is important for masking tapes and taking into consideration the environment can be used. Therefore, at present, the support of a Japanese paper tape for masking use moves to the impregnation-coating type Japanese paper tape base material.
The Japanese paper means a thin, high-strength planar paper which is obtained by paper-making by machine. Different from traditional Japanese papers, chemically cooked pulps such as wood kraft pulps are mainly used as a material of Japanese paper used for the tape base material. However, fibers of paper mulberry (Broussonetia kazinoki x B. papyrifera), Oriental paper bush (Edgeworthia chrysantha), Manila hemp, or the like may also be used. In addition, in order to improve the strength, it is also possible mix synthetic fibers such as polyester fibers, vinylon fibers, etc. Though there is no distinct definition for the machine-made Japanese paper, the thickness is approximately not more than 100 μm, the basis weight is approximately not more than 60 g/m2, and the density is approximately not more than 0.7 g/cm3.
As for the impregnation-coating type Japanese paper masking tape, so far as a crepe paper masking tape is concerned, not only the base material is thick so that the painting becomes thick, but the painting edges are blurred due to unevenness of the crepe, and therefore, beautiful finishing is not achieved. On the other hand, a masking tape using a Japanese paper as a base material is thin and smooth as compared with the crepe paper, and therefore, a parting line of the painting can be beautifully finished. Since the impregnation-coating type Japanese paper masking tape is easily torn off by hand (has hand-tearing properties), a blade is not required at the time of cutting, and its workability is good. In addition, while the impregnation-coating type Japanese paper masking tape has hand-tearing properties, it has sufficiently strength, and its workability is good. However, so far as the easiness of stretching is concerned, since wrinkles are present on the base material, the masking tape using a Japanese paper as a base material is inferior to the easily stretchable crepe paper masking tape.
In the masking tape, if the base material is hardly stretchable, the conformability becomes deteriorated, and at the time of sticking onto an uneven surface or curved surface, lifting or peeling from an adherend is easy to occur, and hence, the workability becomes deteriorated. Examples of a countermeasure for improving the conformability include the use of a base material which is stretchable at a low tension. In a base material in which even if a breaking elongation thereof is large, a large force is required for stretching, the conformability does not become good.
Examples of a method of obtaining a base material which is easily stretchable at a low tension include mixing of synthetic fibers which are easily stretchable at a low tension in the Japanese paper. However, in order to make the base material sufficiently easily stretchable, if the mixing amount of the synthetic fibers is increased, not only tangling among the synthetic fibers is easy to occur in the paper-making of Japanese paper, but the costs increase. Thus, the amount of the synthetic fibers is limited. Furthermore, if the mixing ratio of the synthetic fibers is increased, the stiffness of the base material becomes strong due to the synthetic fibers which are more rigid than natural pulps, and sticking onto the uneven surface becomes difficult. In addition to that, there is a concern that the tear strength becomes too high, so that the hand-tearing properties become deteriorated.
In the masking tape, if the strength of the base material, particularly the tear strength is low, the tape is torn at the time of peeling, thereby resulting in causing a reduction of the work efficiency.
As a countermeasure therefor, synthetic fibers of vinylon, PET, etc. having a fiber length of 3 mm or more are mixed in the Japanese paper tape base material, thereby increasing the tear strength. However, such synthetic fibers are expensive as compared with natural fibers. In order to increase the strength of the base material, there are a method of using synthetic fibers having a longer fiber length and a method of increasing the amount of synthetic fibers. However, if the fiber length is made long, tangling is easy to occur, and hence, there is a limit in the manufacture. If the amount of the synthetic fibers is increased, not only tangling is easily generated in the paper-making of Japanese paper, but the costs increase. Thus, the amount of the synthetic fibers is limited, too.
Japanese Patent No. 4456662 proposes an easily stretchable tape base material with high conformability by mixing a natural pulp with specified polyvinyl alcohol (vinylon) fibers having a low modulus and a large breaking elongation, in which only the fiber surface is dissolved by a wet heat treatment. Japanese Patent No. 4456662 describes that a mercerized pulp may be used for the natural pulp. However, this patent document does not give any specific description regarding its blending amount or effects and does not mention that the tear strength or conformability is enhanced by the mercerized pulp. In addition, if a large amount of synthetic fibers which are dissolved even partially is added, staining of a dryer is caused in a paper-making step, resulting in causing a problem from the standpoint of production.
JP-A-2010-203006 proposes to impregnate a specified resin in a base paper in which a natural pulp is mixed with synthetic fibers having a fineness of from 1 to 5 dtex, thereby improving the tear strength of a base material. However, JP-A-2010-203006 does not mention the conformability. In general, if the fiber diameter of synthetic fibers is large, the rigidity of the fibers becomes high. If a mixing ratio of such synthetic fibers is increased, the stiffness of the base material becomes strong, and the adhesion to the uneven surface becomes inferior. In addition, fibers having high rigidity involve such a defect that tangling is easy to occur in a paper-making step.
Japanese Patent No. 3691622 is concerned with a double-sided tape base material having a mercerized pulp mixed therein. The double-sided tape base material is used upon permeation of an adhesive, whereas the impregnation-coating type tape base material according to the present invention is one prepared by impregnating and coating a resin and then coating an adhesive on one surface, and therefore, the both are different in the configuration and use from each other. Japanese Patent No. 3691622 insists that by using the mercerized pulp, the strength of the double-sided tape base material and the permeability of the adhesive can be maintained while suppressing the use amount of rayon. However, Japanese Patent No. 3691622 does not describe the elongation of the base material.
JP-A-2007-31552 proposes that in order to prevent the occurrence of tape tearing at the time of peeling, a film base material having high tear strength is used for a masking tape. It is difficult to cut this film base material by hand, and a blade is required to be used for cutting, and hence, the workability is inferior.
Japanese Patent No. 4808987 proposes to prepare a fiber product of a Japanese paper tone having excellent tear strength by laminating a nonwoven fabric on a base cloth composed of thermoplastic polymer fibers having a fineness of from 5 to 60 dtex. In Japanese Patent No. 4808987, it is pointed out that in order to obtain the high tear strength, the fineness is required to be 5 dtex or more. Furthermore, similarly, in JP-A-6-200462, it is also pointed out that in order to keep the strength, a fineness of a certain value or more is required. Though a product using a nonwoven fabric is excellent in the strength, not only it is inferior in the hand-tearing properties to a paper base material, but the costs become high.